Irrigating based on environmental conditions saves water and money

Smart irrigation is providing water only when plants need water. PHOTOS COURTESY OF TORO.

Supplemental irrigation is almost always required if your goal is to provide your clients with great lawns. Although many areas of the country have enough rainfall to keep lawns alive, consistent moisture is needed for optimum turf performance. In addition, with the right irrigation scheduling, turf can actually withstand drought periods better, as a larger root mass means a larger capacity for water storage and absorption.

However, there are few more ludicrous sights than sprinklers running at full bore during a downpour. This is a huge waste of water, it’s costly and it’s not good for the turf. Another irrigation no-no is water spraying into the street during high winds. That too can cause problems. For example, if the temperature drops unexpectedly the misdirected irrigation water can create icy slick conditions on a road.

Welcome to the world of irrigation sensors. There is a sensor that can help maximize every drop of water whether your system is simple or sophisticated. “Smart” controllers generally feature sensors in a wide array of configurations, but they can also be added to most existing systems at a reasonable cost.

From the ground up

The most obvious type of sensor measures moisture in the soil. This technology was first developed for agricultural crops. Turfgrass is an ideal landscape element for moisture sensor use as it is generally a uniform planting. In the case of other landscape plants, established trees and shrubs would have different water requirements than an annual flower bed, as would shade-loving perennials as opposed to full-sun groundcovers.

There are two ways that soil moisture sensors work. The first is called “volumetric” measurement. These sensors estimate the volume of water in a given sample of soil. While sophisticated, very accurate neutron probes are used for agricultural sites, most turf and landscape volumetric sensors use what are called “dielectric” techniques.

While this may sound intimidating, a soil’s dielectric constant is simply the soil’s ability to transmit electricity. The wetter the soil, the faster the electricity moves. (Think of a radio dropped in a bath tub and you get the picture!).

Turfgrass, trees and seasonal color all require different amounts of water. Sensors can aid irrigation of these areas.

Volumetric sensors measure the dielectric constant different ways. One of the most commonly seen probes is long and flat and measures the time it takes for an electric charge to move along buried steel rods (wave guides) and the soil around them. Both are buried about 3 inches below the soil surface. Probes with exposed wave guides are installed horizontally with the wide side facing up; those with encased guides are also installed horizontally, but the thin side faces up.

Tensiometrics, the second type of sensor method, have a porous material in contact with the soil that correlates with the amount of water in the soil. Tensiometers measure actual soil water tension, which is the amount actually available to the plant. They are generally used for agriculture operations.

Another related type of sensor uses nodes that contain porous material with embedded electrodes that measure the moisture content of the material after irrigation. This correlates to the moisture in the soil.

Whatever the sensor, when installed with an irrigation controller, when a certain moisture level is reached, a switch opens that overrides the scheduled irrigation cycle. The controller or sensor system has an adjustable setting that allows the user to determine when the soil needs more water for optimum plant growth. This way, instead of the controller activating the system after a heavy rainfall, the system will “wait” until the next irrigation cycle. If the soil is still wet, it will continue to bypass the scheduled irrigation event until the irrigation threshold is reached.

Many soil moisture sensors can be added to existing irrigation systems. They are an excellent addition to a system that already has a controller that has been installed correctly for optimum efficiency.

Smart sensing

When installing a new system, some smart controllers use moisture sensors to bypass irrigation cycles. Controllers can use moisture sensors as an “on demand” trigger. This way, programming is left to the controller and sensor, and irrigation only occurs when the soil moisture is depleted. In order to avoid unexpected surprise cycles (such as activation during times when the turf might be in use for athletics or play), the controller can be set to only water in the evening or off hours.

Smart controllers also use weather data to irrigate at proper times. This is not strictly sensor technology, as the controller is programmed with soil type, plant type and exposure and uses real-time weather information to determine irrigation cycles.

While irrigation based on soil moisture can be optimum for turfgrass, there are other times when an irrigation cycle might be delayed or bypassed for weather-related reasons.

High winds can blow spray patterns to oblivion and result in sidewalks and streets getting the bulk of the water that was meant for the turf. In addition, high winds (especially when temperatures are high) can result in rapid evaporation.

Wind sensors can delay or bypass irrigation cycles until better conditions come about. An anemometer is a device that catches the wind and measures its speed. Cup anemometers use three or four hemispheric cups that are mounted on horizontal arms. The air moves the cups around and the wind speed is calculated depending on how quickly the arms spin. Vane anemometers look like small fans. Most, if not all, landscape irrigation wind sensors use the cup-type anemometer.

Wind sensors are set to bypass the irrigation system when winds reach a certain speed. Depending on the controller, they can be programmed to either resume irrigation as soon as the winds slow to a set threshold or the sprinklers can activate at a specific time chosen by the user.

When it’s raining

It’s fairly obvious what a rain sensor does. It simply stops the system when there is precipitation, which is why they are sometimes called rain shut-off devices. Newer controllers have a special connection that makes it easy to directly install a rain shut-off, but even older controllers can have a sensor hard-wired into the controller.

Although there are rain sensors that work with open catch basins that work by water weight or electrical conductivity, they are prone to having debris or other weight result in false readings and are require water to be removed after a rain event. The most popular devices use expansion disks, which expand when wet and trigger a pressure switch. The amount of rain required to bypass an irrigation cycle can be easily adjusted.

In Florida, all landscape irrigation systems are required to have rain sensors, and municipalities in many other states have enacted similar requirements.

If you want to offer sensor installation as an add-on for your business, you will need to familiarize yourself with specific installation instructions for each sensor. However, there are a few general rules that apply to rain sensor placement.

The sensors should be placed in open areas where they where they can collect rainfall without obstructions. They should not be placed near sprinkler sprays. They are often installed near a roofline (but be sure it is not in contact with water running off the roof). The closer the sensor is to the irrigation controller, the less chance there is for wire breaks.

Keep in mind that the water in the sensor will dry out faster if it is in a sunny, hot spot than if it is shade. You will probably have to experiment with adjusting the thresholds for irrigation bypass and starts, and might need to adjust it seasonally, as well.

One last common sensor monitors the temperature and shuts off the system in the event of freezing temperatures. These are generally used in warmer climates where systems are not drained and shut down during the winter months. These are relatively inexpensive insurance to prevent plant damage and “icicles” in irrigated landscapes.

Newer smart controllers often feature most of the sensors above, but even if you are working with older systems, sensor installation and maintenance is a simple add-on that can increase your bottom line. In addition, your clients will save water and money. Sensors make sense!

Helen M. Stone is a freelance writer specializing in commercial turf and landscape on the West Coast.